The bulls-eye mid-frequency impedance probe has probe electrodes of a
larger surface area that conform to the test surface to provide a very low
impedance coupling between the probe and surface, unlike other open end
coaxial probes. This probe provides accurate measurement of the dielectric
properties of dielectric materials in the mid-frequency range of 10 MHz to
1 GHz on flat and curved solid surfaces, while conforming simple or
complex curved surfaces by providing a resilient means between the probe
contact surface and the probe box.

What is claimed and desired to be secured by Letters Patent of the United
States is:

1. A device for measuring the dielectric characteristics of material
samples by contacting the surface of the samples, said device including a
probe connected to a coaxial like by a connector having an outer conductor
and a central conductor, wherein the probe comprises:

a flexible contact surface comprising a flexible nonconductive substrate,
an external conductor mounted upon the substrate and an internal conductor
mounted upon the substrate, said external and internal conductors adapted
to establish contact with and to conform to a shape of the samples, said
external conductor being in electrical contact with the outer conductor of
the connector and said internal conductor being in electrical contact with
the central conductor of the connector; and

a probe housing in physical contact to the substrate of the flexible
contact surface.

2. The device according to claim 1 in which said substrate is circuit board
partially covered in a metal coating in two separated regions to form said
external conductor and said internal conductor.

3. The device according to claim 1 in which said probe housing makes
physical contact at the outer edges of one central axis of said flexible
contact surface allowing deformation along simple curved surfaces.

4. The device according to claim 1 in which said probe housing makes
physical contact with a resilient means allowing deformation of the
contact surface on compound curved surfaces.

5. A method for measuring dielectric characteristics of a solid material
including the steps of:

calibrating a signal processing device having an electrical line;

measuring a complex impedance of a bulls-eye dielectric probe attached to
the electrical line, said measurement of the complex impedance of the
bulls-eye dielectric probe comprising:

determining if the solid material is covered with a dielectric material;

measuring the complex impedance of the bulls-eye dielectric probe based
upon a dielectric spacer if the solid material is covered with a
dielectric material; and

measuring the complex impedance of the bulls-eye dielectric probe based
upon the presence of an air gap if the solid material is free of
dielectric material;

measuring a complex impedance of the solid material; and

calculating a dielectric constant of the solid material based on an
analytical model of a coaxial cable radiating into semi-infinite space
taking into account the complex impedance of the bulls-eye dielectric
probe and the solid material.

6. The device according to claim 2 in which the internal conductor region
is circular and the external conductor region is a ring surrounding the
internal conductor region.

7. The method of claim 5 in which the electrical line is a coaxial cable.

8. The method of claim 7 in which the signal processing device is a vector
impedance meter.

9. The method of claim 8 in which the step of calibrating the vector
impedance meter with coaxial cable is comprised of the following steps:

connecting coaxial cable to the vector impedance meter;

calibrating vector impedance meter with an electrical short load;

calibrating vector impedance meter with an electrical open load;

calibrating vector impedance meter with a known electrical load.

10. The method of claim 9 in which the step of calibrating the vector
impedance meter with the bulls-eye dielectric probe connected to the
coaxial cable is comprised of the following steps prior to testing
materials having a dielectric spacer:

11. The method of claim 9 in which the step of calibrating the vector
impedance meter with the bulls-eye dielectric probe connected to the
coaxial cable is comprised of the following steps prior to testing
materials not having a dielectric spacer:

The field of the present invention is surface probes capable of
characterizing the dielectric properties of a material, especially test
specimens having a curved surface.

BACKGROUND OF THE INVENTION

Methods of characterizing the electrical dielectric characteristics of
large, curved solid surfaces have lacked an accurate means of testing at
mid-range frequencies.

Information about the complex dielectric properties at mid-range
frequencies is important because the manufacturing tolerances at these
frequency ranges is difficult to control and such properties can vary
widely. Moreover, variations in application of such materials can change
the dielectric properties. Non-destructive testing of such applications
with possibly large curved surfaces requires a portable probe adjustable
to a range of materials and contours.

It is known at high frequencies that electromagnetic radiation can be
directed at a sample from a transmitting horn antenna to a receiving horn
antenna, with the characteristics determined by the change in transmission
from the calibration condition to no test specimen in between to when the
test specimen impedes the transmission.

It is also known that open-ended coaxial-line probes can be inserted into
liquid materials in order to test dielectric properties at mid-range
frequencies, 10 MHz to 1 GHz This technique is described in T. P.
Marsland, "Dielectric measurements with an open-ended coaxial probe", IEE
Proceedings, Vol. 134, Pt. H, No. 4, pp. 341-348, August 1987.

Attempts have been made to make probes that will test curved solid
surfaces, such as in U.S. Pat. No. 5,541,522 by Rosen et al. Such probes
rely upon slender, spring-loaded pins extending out to make contact with
the test surface. The accuracy of such probes is limited because the small
contact area of the pins provides poor, high-impedance coupling to the
test source. The coupling also changes with surface curvature.

SUMMARY OF THE INVENTION

This invention satisfies the above needs. The probe electrodes have a
larger surface area and conform to the test surface to provide a very low
impedance coupling between the probe and surface.

The bulls-eye mid-frequency impedance probe has probe electrodes of a
larger surface area that conform to the test surface to provide a very low
impedance coupling between the probe and surface, unlike other open end
coaxial probes. This probe provides accurate measurement of the dielectric
properties of dielectric materials in the mid-frequency range of 10 MHz to
1 GHz on flat and curved solid surfaces, while conforming to simple or
complex curved surfaces by providing a resilient means between the probe
contact surface and the probe box.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side-view depiction of bulls-eye probe apparatus being used on
a curved test specimen.

FIG. 2 is a functional schematic of bulls-eye probe and coaxial conductor.

FIG. 3 is a front-view depiction of contact surface of bulls-eye probe.

FIG. 4 is a top-level flow diagram of bulls-eye probe method.

FIG. 5 is an expanded flow diagram of bulls-eye probe method.

FIG. 6 is a depiction of electrical flux lines when using bulls-eye probe
on test specimen.

FIG. 7 is a table of the experimental results using the prototype bulls-eye
probe device and method, including use of the EPROBE BASIC computer
program.

FIG. 8 is an equivalent circuit model of the probe with a test specimen.

FIG. 9 is the equivalent circuit model for the probe interacting with a
test specimen covered in a dielectric spacer.

FIG. 10 is an additional table of the experimental results using the
prototype bulls-eye probe device and method, including use of the EPROBE
FORTRAN computer program.

DETAILED DESCRIPTION OF THE INVENTION

The present invention now will be described more fully hereinafter with
reference to the accompanying drawings, in which a preferred embodiment of
the invention is shown. This invention may, however, be embodied in many
different forms and should not be construed as limited to the embodiments
set forth herein; rather, this embodiment is provided so that this
disclosure will be thorough and complete, and will fully convey the scope
of the invention to those skilled in the art. Like numbers refer to like
elements throughout.

Referring to the front view in FIG. 3, the key to the structure of the
bulls-eye mid-frequency dielectric probe is the contact surface (13) which
has an internal conductor (12), a relief ring etched into conductive
coating on circuit board (11); and an external conductor (10).

Referring to the side view in FIG. 2, the contact surface is functionally
related electrically to a coaxial cable as follows. The internal conductor
(12) and external conductor (10), both shown in cross section, are held in
relative position by being attached to a flexible nonconductive substrate
(18), such as a flexible circuit board. The external conductor (10) makes
electrical connection via an external conducting outer connection (23) to
coaxial outer conductor (21) of the coaxial cable (24). The internal
conductor (12) makes electrical connection via a inner conducting outer
connection (17) to the coaxial central conductor (19) of the coaxial cable
(24).

In the preferred embodiment shown in FIG. 1, the prototype bulls-eye probe
has the flexible contact surface (13) is rigidly attached at opposite
edges to the probe box (32). This allows the contact surface (13) to adapt
to a simple curved surface by placing the rigidly attached edges of the
contact surface (13) in line with the non-curved direction of a convex
surface and in line with the curved direction of a concave surface.

The inventive contact surface of the probe provides unique electric field
lines, shown in the side view depiction of FIG. 6. Electric fields flux
lines (41) extend from the center of the bull's eye contact surface (13)
into the dielectric being measured and back to the outer ring of the
bull's eye. There are similar field lines on the other side, the side that
is enclosed within the metal probe box (32) that serves as a shield and a
handle for the operator. In the metal probe box (32) there are additional
field lines from the center of the bull's eye to the walls of the box.

It is anticipated that the attachment between the flexible contact surface
(13) and the probe box (32) could be made with a large number of ways
allowing the probe to be used on complex curved surfaces. For example, a
compressible material or an arrangement of mechanical springs could be
placed between the flexible contact surface (13) and the probe box (32).

It is further anticipated that a number of connecting wires and connection
means to the signal processing device could be used in addition to coaxial
cable.

It is also anticipated geometric shape and orientation of the internal
conductor (12) and external conductor (13) could take many different forms
and dimensions, as well as from other materials than a copper coated
circuit board with an etched portion.

It is additionally anticipated that the probe box could be replaced with
many means and shapes of transferring physical force to the flexible
contact surface such that the contact surface adapts to the shape of the
sample. Moreover, the probe box could be eliminated allowing force
directly to the contact surface, although this is not the preferred
embodiment given the electrical insulation requirements and vulnerability
of electrical conductors.

FIG. 8 shows the equivalent circuit of probe. C.sub.d is the capacitor that
results from the fields in the dielectric being measured. It changes as
the probe contacts different dielectric materials. A quasi-static analysis
shows that the dielectric constant of the material under test,
.epsilon..sub.d, is linearly related to the value of the capacity C.sub.d.
That is,

C.sub.d =A.sub.1.epsilon..sub.d

Where A.sub.1 is a constant determined by the geometry of the probe.

The present invention also includes a method for employing a dielectric
probe such as the apparatus discussed above. As summarized in FIG. 4, the
inventive method includes the steps of (a) calibrating a signal processing
device such as a vector impedance meter with the coaxial cable (101); (b)
measuring the complex impedance of the bulls-eye probe (102); (c) and
measuring the complex impedance of the test specimen (103). Each of these
steps is explained in greater detail in FIG. 5 and the description below:

The first major step of the method is the Electrical Cable Calibration
Procedure (101) which includes the following steps: connecting the
open-ended coaxial cable to signal processing device such as a vector
impedance meter (111); calibrating the signal processing device with the
probe-end of the coaxial cable shorted, such as with a conductive plate
(112); calibrating the signal processing device with the probe-end of the
coaxial cable open (113); and calibrating the signal processing device
with a known impedance load (114).

In actual operation, the complex impedance, Z.sub.m, is measured at the
connector. This measured impedance, Z.sub.m, includes the effects of
C.sub.d, C.sub.f and the length, L, of the connecting wire. The effects of
C.sub.f and L are removed from the measurement to accurately determine
C.sub.d. This is accomplished by performing a calibration as explained
below.

The electrical length of the connecting wire from the probe connector to
the probe surface is determined by placing the probe surface in intimate
electrical contact with an electrically conducting surface and measuring
the impedance, Z.sub.short, at the connector. The impedance, Z.sub.short,
is simply that of a shorted transmission line, namely,

Z.sub.short =jZ.sub.0 tan(2.pi.f L/c).

Where j=-1, Z.sub.0 is the characteristic impedance of the line, c is the
velocity of light, f is the frequency of measurement and L is the
electrical length of the line. Since Z.sub.0, c and f are known and
Z.sub.short is measured, L can be determined. For subsequent computations
it is sufficient to determine .theta..

.theta.=2.pi.f L/c=atan(Z.sub.short /jZ.sub.0)

Again referring to FIG. 5, the second major step is the Probe Calibration
Procedure (102) which begins with connecting the probe end of the coaxial
cable to the dielectric probe (115). It continues depending on whether or
not the test specimen has a dielectric coating or spacer (116). If it does
not, measurement is taken of the probe exposed to air (117). If it does,
measurement of complex impedance of the probe/cable combination is taken
with probe contact surface against an air-backed dielectric sheet
calibration sample of the spacer (118). Then measurement of complex
impedance of the probe/cable combination is taken with the probe contact
surface against a metal-backed dielectric sheet calibration sample of the
dielectric spacer (119). Whether or not the test specimen has a dielectric
coating, the subsequent step is to measure the complex impedance of the
probe/cable combination shorted on a metal block (120).

The fringe capacity, C.sub.f, in parallel with the capacity, C.sub.air, of
an air dielectric (.epsilon..sub.d =.epsilon..sub.air =1) is determined by
measuring the impedance, Z.sub.air, at the connector while holding the
probe in the air away from objects. The measured value of Z.sub.air
includes the effect of the transmission line of length L. Applying the
correction for line length,
##EQU1##

Since this is the capacity of C.sub.f and C.sub.air in parallel,
##EQU2##

The third major step is to measure the complex impedance of the test
specimen (103). The probe is place in intimate contact with an unknown
material whose dielectric constant is to be measured and Z.sub.unk is
measured at the connector. This impedance is corrected for line length, L.
##EQU3##

Since this is the capacity of C.sub.f and C.sub.d in parallel,
##EQU4##

This impedance, Z.sub.unkCorrected, includes the shunt impedance of the
fringe capacity C.sub.f that must be removed to determine C.sub.d. This is
done as follows.

This information allows for the correction of measured data to determine
unknown dielectric constant: We know that

Determination of A.sub.1 : The value of the A.sub.1, the constant based on
the geometry of the probe, can be experimentally determined or it can be
computed using analytical or numerical techniques. N. Marcuvitz presents
one form of an analytical solution in his book, Waveguide Handbook,
McGraw-Hill, 1951, page 213. His analysis is for a coaxial cable radiating
into semi-infinite space, which is close to the geometry of the bull's eye
probe.

The calculations provided above mathematically lend themselves to being
implemented in a computer program. In the preferred embodiment, a BASIC
computer program called EPROBE incorporates the functions of taking inputs
of material parameters and processing probe signals. EPROBE analysis also
includes the effect of a finite thickness dielectric part being measured.
It also includes the effect of an air gap or dielectric spacer between the
probe and the dielectric part being measured, as shown in FIG. 7.

Correction for Dielectric Spacer between Probe and Material being measured:
In some field measurements a thin dielectric sheet may cover the material
being measured. One way of representing this is shown schematically in
FIG. 9. The capacitor C.sub.s represents the series capacitance caused by
the dielectric cover and C.sub.p represents the parallel capacitor caused
by the dielectric cover. These capacitors can be included in the
analytical calculation of the probe constant A.sub.1 or a correction can
be done during the measurement if a sample of the cover sheet is
available. The value of C.sub.p in parallel with C.sub.air and C.sub.f is
determined during calibration by placing the sample cover sheet against
the probe when making the air calibration. The value of C.sub.s is
determined during calibration by placing the sample cover sheet between
the probe and a conducting surface. During the data analysis, the measured
impedances are corrected for C.sub.p and C.sub.s.

The Rosen patent, U.S. Pat. No. 5,541,522 mentioned above, has slender,
spring-loaded conducting pins that extend from the face of their probe and
provide a small area of contact with the test surface. This small area
causes a series impedance that is similar to that of the series capacitors
C.sub.s discussed above for the dielectric cover sheet. In the instant
invention, even with no dielectric spacer, there will be a small air gap
between the probe and test surface, resulting in a series capacitance,
C.sub.s. However, the error caused by C.sub.s is negligible because in the
instant invention the surface area of the probe is large enough that the
impedance of the resulting series capacitance C.sub.s is much smaller than
the impedance of the capacitance, C.sub.d, being measured. In the Grumman
invention, the surface area of the pins is small so the impedance of the
resulting series capacitance C.sub.s can be large comparable to the
impedance of the capacitance, C.sub.d, being measured. This can cause
significant measurement errors, especially when C.sub.s changes with a
change in surface curvature.

The inventive probe and process was used in a series of tests to confirm
its performance to the predicted analytical model. Referring to FIG. 7, a
number of materials were tested using the bulls-eye probe device and the
EPROBE BASIC computer program.

An additional test sample set is given in FIG. 10. The two sets of data in
the sample data file are for (1) using the probe directly on the surface
of a sample and (2) using the probe on the surface of a 0.040" thick glass
epoxy spacer placed on the surface of the sample. The two sets of answers
give an idea of the repeatability. Independent measurement of the
dielectric constant of Teflon was 2.04, so the probe measurement of
2.11-j0.019 and 2.14-j.045 is satisfactory for Teflon. Likewise Nylon was
independently measured to be 3.10-j0.16, so 3.08-j.16 and 3.11-j.19 is
satisfactory. The alumina was not independently measured, but was expected
to be between 8 and 9; it was 8.16-j.15 and 8.56-j.22, which is also
satisfactory.

In the drawings and specification, there has been set forth a preferred
embodiment of the invention, and, although specific terms are employed,
the terms are used in a generic and descriptive sense only and not for
purposes of limitation, the scope of the invention being set forth in the
following claims.